This invention relates to a semiconductor memory device, more particularly to an improvement applicable to a semiconductor memory device comprising a plurality of so-called 1-transistor and 1-capacitor type memory cells.
In the prior art, a so-called channel stop system is employed for isolating the memory cells from each other. Namely, each memory cell is surrounded by a high impurity concentration area which is called a channel stop area, selectively formed in a surface portion of a semiconductor substrate. To keep self alignment for each layer of a semiconductor memory cell, the channel stop process available in the prior art comprises a step of covering the active area, including transistor area of a memory cell, with a mask of silicon nitride, a step of selectively introducing an impurity into an area uncovered by the mask, and a step of growing a thick field silicon oxide layer except on the mask.
From a practical viewpoint, however, the conventional channel stop process using a thick field oxide layer, inevitably has a drawback in that the area of the active area is unintentionally decreased, because the field silicon oxide layer is inclined to grow toward the active area and a lengthy heat treatment causes the impurity doped for the purpose of the channel stop system to diffuse toward the active area.
It is quite natural that a capacitor formed with a silicon oxide layer grown on a substrate and a conductive layer, grown on the silicon oxide layer and a high impurity concentration layer doped beneath the silicon oxide layer spreading beneath the high impurity concentration layer at an interface or p-n junction formed between the substrate and the high impurity concentration layer, is useful to decrease the size of the capacitor for the memory cell. To increase the capacitance of the latter capacitor, for reducing the capacitor size, another high impurity concentration layer with the same conductivity as that of the substrate is interleaved between the first high impurity concentration layer and the substrate.
This method, however, has a drawback in that the above mentioned additional high impurity concentration layer with the same conductivity as that of the substrate, reaches the gate of a transfer gate transistor, thereby increasing the threshold voltage Vth of the same transistor.
Further, in order to prevent the inversion of conductivity from occuring at the surface of the substrate in the field area, a so-called field shield system can be employed rather than some of the conventional means used for the same purpose, such as the channel stop system. This field shield system, however, has a drawback in that it tends to cause a lesser degree of circuit integration, due to the additional masking process required to form the pattern of the field shield layer. Moreover, unlike the channel stop diffusion system, the field shield system is ineffective to prevent the surface inversion originated from the characteristics of the insulator layer thereunder.